This invention relates to an image reading apparatus for use in a digital copy machine and a facsimile machine.
In the field of an image reading apparatus such as an image scanner for reading a document image with use of a line sensor consisting of densely arranged photoelectric conversion elements (such as CCD: Charge Coupled Device), it is set such that the document image is illuminated by a light source and light reflected from the document image is led to the line sensor via an optic system to obtain a light receiving data. It is common practice to perform a shading correction of the light receiving data to correct its deviation caused by a sensitivity dispersion among each of the photoelectric conversion elements and an illumination dispersion.
This shading correction is to be performed before reading the document image for correcting the light receiving data with use of a white reference data obtained from the photoelectric conversion elements upon receiving reflected lights from a white reference plate and a black reference data obtained from the photoelectric conversion elements when the line sensor is shaded.
However, it is not always possible to perform an optimum shading correction as it is likely the case such as the white reference plate is not in a mint condition or electrical noise maybe picked up, rendering to degrade a reliability of the white reference data and the black reference data. This in turn results in disabling the proper shading correction. In order to cope with this problem, it was proposed an image reading apparatus in Japanese Patent Publication No. 2505906 (hereinafter referred to as JP 2505906) in that plural line data of reading the white reference plate are to be taken to lessen an adverse effect due to the blemish of the white reference plate.
With the image reading apparatus of JP 2505906, when an n-th line data of the white reference data is taken, this n-th line data and the data taken prior to this n-th line data obtained by reading the white reference plate are summed and averaged and the resultant data (averaged value) is stored in a memory. As a result, this described way has at least an advantage in view of the necessary memory capacity over another conventional image reading apparatus in that plural line data are stored and summation of all the data and an average value are calculated.
With the image reading apparatus of JP 2505906, the average value is calculated with the most recent line reading data of the white reference plate and the data stored in the memory that is an average value between the previous data and the data stored before this previous data is taken. Thus the data (x1) read at the beginning is blended with that of the second data (x2) to obtain the average value. This average data is a resultant of the simple mathematical calculation of the two data, i.e., (x1+x2)/2=XI. However, when the next data is taken (let's say x3), then the average value is to be (XI+x3)/2=XII. Then the resultant averaged data (let us call this average a weighted average XII) is further blended with the next data. Accordingly the effect of the first data (x1) gets more and more insignificant to the resultant data as the data is summed and averaged with the following data. The last data (xn) is most significant to the resultant data because of this weighed average method utilized in JP 2505906. Thus let us imagine the case such as a portion of the white reference plate corresponding to the last line data were especially blemished for instance, as the last data (xn) is highly significant to the resultant data then the resultant data could not represent the real character of the white reference plate; it would rather contain a great error.